The present invention relates to systems for determining the focus condition of a lens, controlling the focus of the lens, and finding a range to an object.
In many imaging applications, it is often desired to focus a lens or other collector on an object and to maintain the lens sharply focussed despite the object's longitudinal motion, i.e., motion along the optical axis of the lens. One such imaging application is eye-gaze tracking in which an apparatus determines the point in space at which the eye is looking.
Prior eye-gaze trackers are disclosed in U.S. Pat. Nos. 3,864,030 to Cornsweet; 4,287,410 and 4,373,787 to Crane et al.; 4,648,052 to Friedman et al.; and in certain U.S. patent applications of Thomas E. Hutchinson, Ser. Nos. 07/086,809; 07/267,266, filed Nov. 4, 1988; and 07/326,787. Those systems typically illuminate the eye with infrared light which is reflected from various parts of the eye, particularly the cornea and retina, to an imaging device such as a videocamera. The spatial relations between the reflections are used to determine the gaze point. For example, the corneal reflection moves about eighty micrometers per degree of eye rotation with respect to the pupil reflection.
From elementary geometry, it will be appreciated that the location finding accuracy of such trackers is heavily dependent on accurately locating the eye reflections with respect to the apparatus and with respect to each other. Thus, the gaze point accuracy can be improved by maintaining the camera sharply focussed on the eye.
One (uncomfortable) way of keeping the camera focussed is by preventing relative motion of the eye and camera, e.g., by restraining the eye or head of the user. Another way is by providing an autofocus mechanism to the camera. (If lateral motions of the eye, i.e., motions perpendicular to the optical axis, exceed the camera's instantaneous field of view, a lateral tracking mechanism is also needed.) The above-cited U.S. patents describe two types of autofocus mechanism whereby longitudinal eye displacements are detected using the corneal reflection of a light source. In the patent to Cornsweet, a variable amplitude modulation due to motion of the source's image formed between two chopper wheels is detected. In the patents to Crane et al., the difference in output between two detectors longitudinally equidistant, when properly focussed, from the source's image is detected. U.S. Pat. No. 3,869,694 to Merchant et al. describes an improved eyegaze tracker that includes an ultrasonic position measuring system that is used to adjust the focus of the tracker.
Other devices for focussing an imaging device on an eye are disclosed in U.S. Pat. Nos. 4,251,139 to Matsumura; 4,626,089 to Takahashi et al.; 4,673,264 to Takahashi; and 4,678,297 to Ishikawa et al. The patents to Ishikawa et al. and Matsumura disclose optical system focussing and alignment by projecting a mark image onto the cornea and detecting the reflected mark image. The patents to Takahashi et al. and Takahashi disclose projecting a mark image into the eye and detecting the mark image reflected from the retina.
Those devices and other mechanisms such as multi-camera-parallax devices are unsuitable for many gaze trackers and other applications because they typically require additional equipment and complex calibration, and may not provide the required range measurement accuracy. In addition, they can excessively restrict the freedom of motion of the user.
Accordingly, it is an object of the present invention to provide an apparatus for determining the focus condition of a lens. It is another object of the invention to provide a head tracker which accurately maintains a clear image of a user's eye for determining the user's gaze point. It is a further object of the present invention to provide a head tracker which accurately determines the range to the user's eye.